Meaning

noun a linear unit of measurement (equal to 6 feet) for water depth

noun (mining) a unit of volume (equal to 6 cubic feet) used in measuring bodies of ore

verb come to understand

originFrom Middle English fathome, fadom, fadme (“unit of length of about six feet; depth of six feet for nautical soundings; (loosely) cubit; ell”) [and other forms], from Old English fæþm, fæþme (“encircling or outstretched arms, bosom, embrace; envelopment; control, grasp, power; fathom (unit of measurement); cubit”) [and other forms], from Proto-West Germanic *faþm (“outstretched arms, embrace;…

Etymology from Wiktionary, CC BY-SA 3.0